WASHINGTON — Only 61% of the more than 100 million customer service phone calls made to the Internal Revenue Service last year were answered, the nation's taxpayer advocate says in her annual report released Thursday.

In addition, the IRS has done a poorer job in recent years dealing with walk-ins at local offices and responding on a timely basis to letters, Taxpayer Advocate Nina Olson reported. She cited an 8% cut in IRS funding and staffing since 2010 as the cause.

As a result, the report says, "Tax preparation and filing assistance is now, for the most part, privatized. That is, for a taxpayer to comply with his or her requirement to file a tax return, the taxpayer must pay for assistance, pay for software and pay for advice.''

Taxpayers, Olson said in an interview, "will not be able to get through on the phone when the IRS has sent them a notice and says, 'We are going to do X to you. Or we have adjusted your return and if you disagree, call us or write us.'''

IRS Commissioner John Koskinen made a public appeal earlier this week for "adequate resources'' for his agency.

"I'm extremely concerned about the deep budget cuts that our agency has had to absorb over the last few years,'' Koskinen said. "It's critical that we find a solution to this problem, and I will do everything I can to make sure that we do.''

The National Treasury Employees Union said the report highlights the need to restore funding to the IRS.

"All the numbers that should be going up — taxpayers helped, calls answered, tax issues resolved, training completed — are going down,'' NTEU President Colleen M. Kelley said in a statement. "All the numbers that should be decreasing are rising — identity theft cases, telephone wait times, delays in answering letters."

The IRS also came under attack last year for targeting Tea Party-affiliated non-profits for special scrutiny in making decisions on tax exemption.

“It’s one of the more dysfunctional branches of government.”

Sen. John McCain, R-Ariz.

"It's one of the more dysfunctional branches of government,'' Sen. John McCain, R-Ariz., said Thursday. "And of course, they spend at least some of their time trying to go after the conservative groups."

Republican Sen. John Cornyn of Texas described the deterioration in IRS customer service as "outrageous.''

"I wish I could say I was surprised, which is part of the reason why giving them additional responsibilities like enforcing Obamacare is a bad idea,'' Cornyn said. "But there are other problems as well. For instance, the largest single source of tax fraud in America is the earned income tax credit.''

The 16-day partial government shutdown in October forced the IRS to delay processing refunds for the 2013 tax year. The agency will start processing returns Jan. 31 — 10 days later than originally planned.

The processing of tax returns a year ago also was delayed — until Jan. 30 — because Congress didn't finish renewing expiring Bush-era tax cuts until New Years Day of 2013. The Bush tax cuts were not renewed for families earning more than $450,000, and many deductions for households earning more than $250,000 were phased out.

Taxpayers with questions about those changes often were unable to get a response from the IRS over the 12-month period ending Sept. 30.

The unanswered phone call rate of 39% is up from 13% 10 years earlier.

For the 61% of callers who did get through to the IRS, the average wait to talk to a representative was more than 17 minutes.

Only 110,000 walk-in customers received answers to tax law questions, which was 685,000 fewer than the 795,000 handled a decade earlier. And the IRS has abandoned helping elderly, disabled and low-income people prepare their returns at those sites.

About 53% of the 8.4 million letters from taxpayers seeking adjustments to their tax liability went unanswered for more than 45 days last year, according to Olson's report. A decade ago, it was 12%.

Olson said money to train IRS customer representatives also was cut, from $172 million in fiscal 2010 to $22 million in fiscal 2013, which increased the possibility that taxpayers "are more likely to receive incorrect information or no information.''

The IRS has announced it plans to stop answering tax law questions after this year's tax filing season ends.